Effects of cellular copper content on copper uptake and metallothionein and ceruloplasmin mRNA levels in mouse hepatocytes

Theranostic Nuclear Medicine with Gallium-68, Lutetium-177, Copper-64/67, Actinium-225, and Lead-212/203 Radionuclides

Molecular changes in malignant tissue can lead to an increase in the expression levels of various proteins or receptors that can be used to target the disease. In oncology, diagnostic imaging and radiotherapy of tumors is possible by attaching an appropriate radionuclide to molecules that selectively bind to these target proteins. The term "theranostics" describes the use of a diagnostic tool to predict the efficacy of a therapeutic option. Molecules radiolabeled with γ-emitting or β+-emitting radionuclides can be used for diagnostic imaging using single photon emission computed tomography or positron emission tomography. Radionuclide therapy of disease sites is possible with either α-, β-, or Auger-emitting radionuclides that induce irreversible damage to DNA. This Focus Review centers on the chemistry of theranostic approaches using metal radionuclides for imaging and therapy. The use of tracers that contain β+-emitting gallium-68 and β-emitting lutetium-177 will be discussed in the context of agents in clinical use for the diagnostic imaging and therapy of neuroendocrine tumors and prostate cancer. A particular emphasis is then placed on the chemistry involved in the development of theranostic approaches that use copper-64 for imaging and copper-67 for therapy with functionalized sarcophagine cage amine ligands. Targeted therapy with radionuclides that emit α particles has potential to be of particular use in late-stage disease where there are limited options, and the role of actinium-225 and lead-212 in this area is also discussed. Finally, we highlight the challenges that impede further adoption of radiotheranostic concepts while highlighting exciting opportunities and prospects.

Peptide Receptor Radionuclide Therapy with 67Cu-CuSarTATE Is Highly Efficacious Against a Somatostatin-Positive Neuroendocrine Tumor Model

Peptide receptor radionuclide therapy (PRRT) using radiolabeled octreotate is an effective treatment for somatostatin receptor 2-expressing neuroendocrine tumors. The diagnostic and therapeutic potential of ⁶⁴Cu and ⁶⁷Cu, respectively, offers the possibility of using a single somatostatin receptor-targeted peptide conjugate as a theranostic agent. A sarcophagine cage amine ligand, MeCOSar (5-(8-methyl-3,6,10,13,16,19-hexaaza-bicyclo[6.6.6]icosan-1-ylamino)-5-oxopentanoic acid), conjugated to (Tyr³)-octreotate, called ⁶⁴Cu-CuSarTATE, was demonstrated to be an imaging agent and potential prospective dosimetry tool in 10 patients with neuroendocrine tumors. This study aimed to explore the antitumor efficacy of ⁶⁷Cu-CuSarTATE in a preclinical model of neuroendocrine tumors and compare it with the standard PRRT agent, ¹⁷⁷Lu-LuDOTA-Tyr³-octreotate (¹⁷⁷Lu-LuTATE). Methods: The antitumor efficacy of various doses of ⁶⁷Cu-CuSarTATE in AR42J (rat pancreatic exocrine) tumor-bearing mice was compared with ¹⁷⁷Lu-LuTATE. Results: Seven days after a single administration of ⁶⁷Cu-CuSarTATE (5 MBq), tumor growth was inhibited by 75% compared with vehicle control. Administration of ¹⁷⁷Lu-LuTATE (5 MBq) inhibited tumor growth by 89%. Survival was extended from 12 d in the control group to 21 d after treatment with both ⁶⁷Cu-CuSarTATE and ¹⁷⁷Lu-LuTATE. In a second study, the efficacy of fractionated delivery of PRRT was assessed, comparing the efficacy of 30 MBq of ⁶⁷Cu-CuSarTATE or ¹⁷⁷Lu-LuTATE, either as a single intravenous injection or as two 15-MBq fractions 2 wk apart. Treatment of tumors with 2 fractions significantly improved survival over delivery as a single fraction (⁶⁷Cu-CuSarTATE: 47 vs. 36 d [P = 0.036]; ¹⁷⁷Lu-LuTATE: 46 vs. 29 d [P = 0.040]). Conclusion: This study demonstrates that ⁶⁷Cu-CuSarTATE is well tolerated in BALB/c nude mice and highly efficacious against AR42J tumors in vivo. Administration of ⁶⁷Cu-CuSarTATE and ¹⁷⁷Lu-LuTATE divided into 2 fractions over 2 wk was more efficacious than administration of a single fraction. The antitumor activity of ⁶⁷Cu-CuSarTATE in the AR42J tumor model demonstrated the suitability of this novel agent for clinical assessment in the treatment of somatostatin receptor 2-expressing neuroendocrine tumors.

The effect of maternal iron deficiency on zinc and copper levels and on genes of zinc and copper metabolism during pregnancy in the rat

Fe deficiency is relatively common in pregnancy and has both short- and long-term consequences. However, little is known about the effect on the metabolism of other micronutrients. A total of fifty-four female rats were fed control (50 mg Fe/kg) or Fe-deficient diets (7·5 mg/kg) before and during pregnancy. Maternal liver, placenta and fetal liver were collected at day 21 of pregnancy for Cu and Zn analysis and to measure expression of the major genes of Cu and Zn metabolism. Cu levels increased in the maternal liver (P=0·002) and placenta (P=0·018) of Fe-deficient rats. Zn increased (P<0·0001) and Cu decreased (P=0·006) in the fetal liver. Hepatic expression of the Cu chaperones antioxidant 1 Cu chaperone (P=0·042) and cytochrome c oxidase Cu chaperone (COX17, P=0·020) decreased in the Fe-deficient dams, while the expression of the genes of Zn metabolism was unaltered. In the placenta, Fe deficiency reduced the expression of the chaperone for superoxide dismutase 1, Cu chaperone for superoxide dismutase (P=0·030), ceruloplasmin (P=0·042) and Zn transport genes, ZRT/IRT-like protein 4 (ZIP4, P=0·047) and Zn transporter 1 (ZnT1, P=0·012). In fetal liver, Fe deficiency increased COX17 (P=0·020), ZRT/IRT-like protein 14 (P=0·036) and ZnT1 (P=0·0003) and decreased ZIP4 (P=0·004). The results demonstrate that Fe deficiency during pregnancy has opposite effects on Cu and Zn levels in the fetal liver. This may, in turn, alter metabolism of these nutrients, with consequences for development in the fetus and the neonate.

Transcriptional regulation of copper metabolism genes in the liver of fetal and neonatal control and iron-deficient rats

Copper and iron metabolism have been known to interact for many years. We have previously shown, during pregnancy, that copper levels in the maternal liver rise as a consequence of iron deficiency, but that levels in the fetal liver decrease. In this paper, we measure expression of genes involved in copper metabolism in fetal and postnatal liver, to test whether alterations can explain this observation. Additionally, we study the extent to which gene expression changes in the latter stages of pregnancy and in the perinatal period. Ctr1 expression levels dropped to term, rising again thereafter. There was no difference in gene expression between control and iron deficient animals. Atox1 expression remained approximately stable until term, and then there was a rise to a maximum at about Day 8. Atp7a expression levels remained constant, except for a brief drop at term. Atp7b levels, in contrast, decreased from a maximum early in gestation to low levels in the term and post-natal livers. Ceruloplasmin expression appeared to be diametrically opposite to Atp7b. The other two metallochaperones showed the same pattern of expression as Atox1, with a decrease to term, a rise at Day 1, or a rise after birth followed by a brief decrease at about Day 3. None of the genes were significantly affected by iron deficiency, suggesting that changes in expression cannot explain the altered copper levels in the fetal and neonatal liver.

Glycosylphosphatidylinositol-linked ceruloplasmin is expressed in multiple rodent organs and is lower following dietary copper deficiency

Ceruloplasmin (Cp), a multicopper ferroxidase, is expressed as both a secreted (sCp) plasma enzyme from the liver and a membrane-bound glycosylphosphatidylinositol-anchored (GPI-Cp) splice variant protein. Cp is thought to be essential for iron mobilization as selective iron overload occurs in aceruloplasminemia in humans and in Cp null mice. Dietary copper-deficient (CuD) rodents have near total loss of Cp activity, severe loss of Cp protein and develop anemia. Hepatic iron augmentation is often observed, suggesting that loss of Cp function may be correlated with anemia. The impact of CuD treatment on GPI-Cp has not previously been evaluated. Our hypothesis was that CuD rodents would have lower levels of GPI-Cp and this would correlate with higher tissue iron retention. In these studies, GPI-Cp was detected in purified membranes of multiple organs of rats and mice but not Cp −/− mice. Immunoreactive Cp protein was released with phosphatidylinositol phospholipase C treatment and expressed ferroxidase activity. Following perinatal and postnatal copper restriction, GPI-Cp was markedly lower in the spleen and modestly lower in the liver of CuD rats and mice, when compared with copper-adequate (CuA) rodents. However, spleen non-heme iron (NHI) was lower in CuD than CuA rats, and not different in CuD mice. Hepatic iron was higher only in CuD mice. Spleen and liver membranes of CuD rats expressed augmented levels of ferroportin, the iron efflux transporter, which may explain lower NHI content in the spleen of CuD rats despite a greater than 50% lower level of the multicopper ferroxidase GPI-Cp. Spleen and liver levels of GPI-Cp mRNA were not impacted in CuD rats, suggesting that turnover rather than biosynthesis may explain the lower steady-state levels of GPI-Cp following dietary copper restriction. Lower GPI-Cp did not correlate with tissue iron retention and thus the role, if any, of Cp in anemia of copper deficiency is unknown.

Levels of plasma ceruloplasmin protein are markedly lower following dietary copper deficiency in rodents

Ceruloplasmin (Cp) is a multicopper oxidase and the most abundant copper binding protein in vertebrate plasma. Loss of function mutations in humans or experimental deletion in mice result in iron overload consistent with a putative ferroxidase function. Prior work suggested plasma may contain multiple ferroxidases. Studies were conducted in Holtzman rats (Rattusnorvegicus), albino mice (Mus musculus), Cp-/- mice, and adult humans (Homo sapiens) to investigate the copper-iron interaction. Dietary copper-deficient (CuD) rats and mice were produced using a modified AIN-76A diet. Results confirmed that o-dianisidine is a better substrate than paraphenylene diamine (PPD) for assessing diamine oxidase activity of Cp. Plasma from CuD rat dams and pups, and CuD and Cp-/- mice contained no detectable Cp diamine oxidase activity. Importantly, no ferroxidase activity was detectable for CuD rats, mice, or Cp-/- mice compared to robust activity for copper-adequate (CuA) rodent controls using western membrane assay. Immunoblot protocols detected major reductions (60-90%) in Cp protein in plasma of CuD rodents but no alteration in liver mRNA levels by qRT-PCR. Data are consistent with apo-Cp being less stable than holo-Cp. Further research is needed to explain normal plasma iron in CuD mice. Reduction in Cp is a sensitive biomarker for copper deficiency.

Gene expression endpoints following chronic waterborne copper exposure in a genomic model organism, the zebrafish, Danio rerio

Although copper (Cu) is an essential micronutrient for all organisms, in excess, waterborne Cu poses a significant threat to fish from the cellular to population level. We examined the physiological and gene expression endpoints that chronic waterborne Cu exposure (21 d) imposes on soft-water acclimated zebrafish at two environmentally relevant concentrations: 8 microg/l (moderate) and 15 microg/l (high). Using a 16,730 65-mer oligonucleotide customized zebrafish microarray chip related to metal metabolism and toxicity to assess the transcriptomic response, we found that 573 genes in the liver responded significantly to Cu exposure. These clustered into three distinct patterns of expression. There was distinct upregulation of a majority of these genes under moderate Cu exposure and a significant downregulation under high Cu exposure. Microarray results were validated by qPCR of eight genes; two genes, metallothionein 2 (mt2) and Na(+)-K(+)-ATPase 1a1 (atp1a1), displayed increased expression under both Cu exposures, indicative of potential genetic endpoints of Cu toxicity, whereas the remaining six genes demonstrated opposing effects at each Cu exposure. Na(+)-K(+)-ATPase enzyme activity decreased during Cu exposure, which may be linked to Cu's competitive effects with Na(+). Whole body cortisol levels were significantly increased in Cu-exposed fish, which prompted an analysis of the promoter region of all significantly regulated genes for glucocorticoid (GRE) and metal (MRE) response elements to dissociate metal- and stress-specific gene responses. Of the genes significantly regulated, 30% contained only a GRE sequence, whereas 2.5% contained only a consensus MRE. We conclude that the indirect effects of Cu exposure regulate gene expression to a much greater degree than the direct effects.

Changes in tubular dysfunction marker levels in parallel with the levels of copper, rather than cadmium, in urine of middle-aged women in non-polluted areas

OBJECTIVE

This study was initiated to investigate if and to what extent the elevations in urinary alpha(1)- and beta(2)-microglobulins (alpha(1)-MG and beta(2)-MG, respectively) are specific to Cd nephro-toxicity.

METHODS

Stored urine samples, collected from 1,000 adult women in 11 prefectures all over Japan, were analysed for eight elements (Ca, Cd, Co, Cu, Mg, Mn, Ni and Zn), alpha(1)-MG, beta(2)-MG, creatinine (CR) and specific gravity (SG); the data were cited from previous publications. The levels of eight elements and two MGs were expressed as observed and after correction for CR or SG (1.016). Age, CR and SG distributed normally, whereas two MGs and eight elements distributed log-normally. The 1,000 cases were classified into quartiles (i.e., 250 cases/group) of the lowest to the highest values (Groups A to D) for each element.

RESULTS

alpha(1)-MG and beta(2)-MG increased as a function of Ca, Cd and Cu when corrected for CR, and the two MGs increased in parallel only with Cd and Cu after SG correction. Prevalence of alpha(1)-MG-uria (cut-off values; 5.3 and 6.5 mg/g cr or l) and beta(2)-MG-uria (cut-off values; 300 and 400 microg/g cr or l) was elevated with increasing Ca, Cd and Cu when corrected for CR, but only with Cd and Cu when corrected for SG. Multiple regression analysis of 353 cases of 50-59 year-old women with the eight element levels as independent variables and one of the two MGs as a dependent variable showed that Cu was the most influential element, and both Cd and Zn were less influential.

CONCLUSIONS

Urinary levels of alpha(1)-MG and beta(2)-MG among women with no environmental Cd exposure correlated more closely with Cu levels than Cd levels in urine. The observation suggests that sub-clinical elevation in alpha(1)-MG and beta(2)-MG is not always attributable to nephro-toxicity of Cd, and that consideration on possible effects of other elements such as Cu is necessary.

Decreased hephaestin activity in the intestine of copper-deficient mice causes systemic iron deficiency

Copper and iron metabolism intersect in mammals. Copper deficiency simultaneously leads to decreased iron levels in some tissues and iron deficiency anemia, whereas it results in iron overload in other tissues such as the intestine and liver. The copper requirement of the multicopper ferroxidases hephaestin and ceruloplasmin likely explains this link between copper and iron homeostasis in mammals. We investigated the effect of in vivo and in vitro copper deficiency on hephaestin (Heph) expression and activity. C57BL/6J mice were separated into 2 groups on the day of parturition. One group was fed a copper-deficient diet and another was fed a control diet for 6 wk. Copper-deficient mice had significantly lower hephaestin and ceruloplasmin (approximately 50% of controls) ferroxidase activity. Liver hepcidin expression was significantly downregulated by copper deficiency (approximately 60% of controls), and enterocyte mRNA and protein levels of ferroportin1 were increased to 2.5 and 10 times, respectively, relative to controls, by copper deficiency, indicating a systemic iron deficiency in the copper-deficient mice. Interestingly, hephaestin protein levels were significantly decreased to approximately 40% of control, suggesting that decreased enterocyte copper content leads to decreased hephaestin synthesis and/or stability. We also examined the effect of copper deficiency on hephaestin in vitro in the HT29 cell line and found dramatically decreased hephaestin synthesis and activity. Both in vivo and in vitro studies indicate that copper is required for the proper processing and/or stability of hephaestin.

Copper overload affects copper and iron metabolism in Hep-G2 cells

Divalent metal transporter #1 (DMT1) is responsible for intestinal nonheme Fe apical uptake. However, DMT1 appears to have an additional function in Cu transport in intestinal cells. Because the liver has an essential role in body Cu homeostasis, we examined the potential involvement of Cu in the regulation of DMT1 expression and activity in Hep-G2 cells. Cells exposed to 10 microM Cu exhibited a 22-fold increase in Cu content and a twofold decrease in Fe content compared with cells maintained in 0.4 microM Cu. (64)Cu uptake in Cu-deficient Hep-G2 cells showed a twofold decrease in K(m) compared with cells grown in 10 microM Cu. The decreased K(m) may represent an adaptive response to Cu deficiency. Cells treated with >50 microM Cu, showed an eightfold increase in cytosolic metallothionein. DMT1 protein decreased (35%), suggesting that intracellular Cu caused a reduction of DMT1 protein levels. Our data indicate that, as a result of Cu overload, Hep-G2 cells reduced their Fe content and their DMT1 protein levels. These findings strongly suggest a relationship between Cu and Fe homeostasis in Hep-G2 cells in which Cu accumulation downregulates DMT1 activity.

Trace elements in human physiology and pathology. Copper

Copper is a trace element, important for the function of many cellular enzymes. Copper ions can adopt distinct redox states oxidized Cu(II) or reduced (I), allowing the metal to play a pivotal role in cell physiology as a catalytic cofactor in the redox chemistry of enzymes, mitochondrial respiration, iron absorption, free radical scavenging and elastin cross-linking. If present in excess, free copper ions can cause damage to cellular components and a delicate balance between the uptake and efflux of copper ions determines the amount of cellular copper. In biological systems, copper homeostasis has been characterized at the molecular level. It is coordinated by several proteins such as glutathione, metallothionein, Cu-transporting P-type ATPases, Menkes and Wilson proteins and by cytoplasmic transport proteins called copper chaperones to ensure that it is delivered to specific subcellular compartments and thereby to copper-requiring proteins.

Placental ceruloplasmin homolog is regulated by iron and copper and is implicated in iron metabolism

We previously reported an endogenous, membrane-bound Cu oxidase with homology to ceruloplasmin in BeWo cells, a placental choriocarcinoma cell line. In this previous study, ceruloplasmin immunoreactivity was localized to the perinuclear region and non-brush-border membranes. Here, we show that azide-sensitive oxidase activity is enriched in the same fractions, correlating subcellular localization of enzyme activity with ceruloplasmin immunoreactivity. Expression of the placental Cu oxidase is inversely proportional to Fe status and directly proportional to Cu status at enzyme and protein levels. To identify a role for the Cu oxidase, cells were exposed to (59)Fe-transferrin for 18 h in an environment of 20% O(2) or 5% O(2). At 5% O(2), Cu-deficient cells retain significantly more (59)Fe than control cells. This excess in (59)Fe accumulation is caused by a significant decrease in (59)Fe release. These results indicate that downregulation of the placental Cu oxidase in BeWo cells impairs Fe release. This effect is only apparent in an environment of limited O(2).

Cu metabolism in the liver

This paper has, given some idea of our concepts of the processes involved in the transport of Cu across cell membranes in the liver, which we have summarised in Fig 1. Cu(II)His2 is reduced to Cu(I). This is transported across the membrane, re-oxidised, either before or after binding to glutathione (Freedman et al., 1989) or HAH1 (Klomp et al., 1997), binds to SAHH, and donates Cu(II) to the ATPase. It is very interesting that cells which are very diverse from an evolutionary point of view still use very similar methods to handle the metal. Whether regulation of transport is also the sam remains to be seen. We would guess that, although there will be strong similarities, there will also be very significant differences, reflecting the different environments seen by different tissues in mammalian cells and given the different requirements of the tissues.

Effect of high dietary zinc on plasma ceruloplasmin and erythrocyte superoxide dismutase activities in copper-depleted and repleted rats

The effect of moderately high dietary zinc (Zn) on the activities of plasma (PL) ceruloplasmin (CP), and PL and erythrocyte (RBC) copper (Cu), Zn superoxide dismutase (SOD) was determined in weanling rats fed Cu-deficient (DEF; < 1 mg Cu/kg), marginal (MAR; 2 mg Cu/kg), or control (CON; 5 mg Cu/kg) copper diets containing normal or high Zn (HZn; 60 mg/kg) for 4 wk and supplemented with oral Cu (CuS; 5 mg/L) in drinking water for 0, 1, 3, or 7 d. PL Cu decreased (67% compared to CON; p < or = 0.05) in the DEF and increased to control level after 3 d of CuS; increased in the MAR group after 1 d of CuS. HZn reduced overall PL Cu by 27% in all groups, but did not alter the linear increase in PL Cu between 0 and 3 d of Cu S. PL CP activity altered concomitantly with PL Cu levels: The time course of increase in CP activity after 0-3 d of CuS was not influenced by HZn in the diet and CP declined in the DEF group by 92%. There was no correlation between dietary Cu level and PL CP. PL SOD activity decreased by 46% (p < or = .05) in the DEF group, increased to control activity after 1 d of CuS and declined slightly after 7 d; MAR diet did not alter PL SOD. HZn diet increased PL SOD activity in all groups by 150%, reduced activity in the DEF and MAR groups by 65 and 37% and delayed the recovery of PL SOD after CuS. RBC SOD declined in the DEF and MAR groups by 56 and 33% (p < or = 0.05) and did not respond to CuS; HZn diet did not influence RBC SOD activity. These data indicate that moderately high Zn in the diet reduces PL Cu, but not PL CP activity or the recovery of PL Cu or CP activity after oral CuS of Cu-deficient rats, modifies the response of PL SOD to dietary Cu, but does not influence RBC SOD activity.

Defective biliary copper excretion in Wilson's disease: the role of caeruloplasmin

Previous studies have failed to explain the link between copper accumulation and abnormal caeruloplasmin expression in Wilson's disease. Furthermore, despite the isolation of a candidate gene for Wilson's disease, which predicts a defective copper transport protein, the localization of this putative protein and its relationship to the pathway involved in copper excretion and to caeruloplasmin remain unknown. We now present evidence that caeruloplasmin, the major plasma copper-carrying protein, is present in the liver in Wilson's disease, and thus that reduced circulating levels of the protein result from a post-translational defect in the secretory pathway. We have also identified a novel form of caeruloplasmin, molecular weight 125 kD, which we propose may act as the carrier for excretory copper into bile, since it is normally present in both liver and bile, although largely absent from serum, and undetectable in bile from Wilson's disease patients. The presence of this form of caeruloplasmin in Wilson's disease liver suggests that a related post-translational defect may also be responsible for its absence from bile in Wilson's disease. This study thus provides the first plausible explanation of a link between the defective copper excretion and the reduced plasma caeruloplasmin, which characterize Wilson's disease.

Response of hepatic function to hepatic copper deposition in rats fed a diet containing copper

Fischer rats were a fed diet supplied with copper chloride (150-600 ppm) for 60 d from weaning. Serum (glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) activities were increased with the increase of Cu concentration in the diet. Biliary excretion of Cu was related to the dietary Cu level. Depositions of hepatic and renal Cu were also related to the dietary Cu level in a dose-dependent manner. In particular, hepatic (155.2 +/- 13.3 micrograms/g) and renal (44.9 +/- 4.4 micrograms/g) Cu concentrations increased abruptly in the Cu-600 ppm group. In the liver, about 60% of Cu was distributed in the soluble fraction (100,000 g supernatant). In the Cu-600 ppm group, 25% of cystosolic Cu was bound to metallothionein (MT). Our results suggest that chronic exposure to Cu appears to have a deleterious effect on the hepatic function, and further, that even in rats with normal biliary Cu excretion, clearance of Cu from the liver may be marginal when dietary Cu is near the 600-ppm level. Although Cu is an essential nutrient, an overload of Cu should be avoided.

Excess copper and ceruloplasmin biosynthesis in long-term cultured hepatocytes from Long-Evans Cinnamon (LEC) rats, a model of Wilson disease

Immortalized hepatic cell lines obtained from laboratory animals or patients with defects in copper metabolism in the liver provide new approaches to examine related metabolism and toxicity. We established a series of hepatic cell lines from the liver of Long-Evans Cinnamon (LEC) rats, using recombinant adenovirus which expresses SV40 large T. Cells from the LEC rats were cultured and accumulated larger amounts of copper than did control cells, when the concentrations of copper in the culture medium exceeded 5 microM. The secretion of ceruloplasmin (CP) from the cultured cells was not reduced in hepatocytes from LEC cells, as compared with the control cells. As accumulation of copper did not affect CP secretion, CP production was not likely to be affected by the accumulation of copper in LEC rat hepatocytes. The production of holo-CP was further investigated by transfection of human CP cDNA and detection of human holo-CP by immunological procedures and use of a monoclonal antibody (mAb CP60) which recognizes human holo-CP but not human apo-CP and rat CP. Hepatocytes from the LEC rats processed and secreted holo-CP into the medium, even with excess copper present in the medium. These observations suggest that the genetic defect in LEC rats did not alter biosynthetic and secretory pathways of CP and that the intracellular copper concentration did not regulate the synthesis and processing of CP in the cultured hepatocytes. Low ceruloplasmin levels are observed in most, but not all, patients with Wilson disease, as well as in LEC rats. Our results do suggest that the copper transporting ATPase encoded in the Wilson disease gene is not a integral part of the biochemical mechanism of copper incorporation into apoprotein. The cell lines and immunological procedures we used are expected to add to information on biologically important process related to copper metabolism and to CP biosynthesis.

A comparison of copper uptake by liver plasma membrane vesicles and uptake by isolated cultured rat hepatocytes

We studied copper uptake from copper dihistidine complexes by plasma membrane vesicles isolated from rat liver and compared the data with those for uptake under the same conditions by hepatocytes cultured from rat liver to determine whether membrane vesicles can be used to study copper uptake. Marker enzyme analysis showed a 28-fold increase in 5'-nucleotidase activity, a slight increase in endoplasmic reticulum and no contamination with mitochondrial membranes. Copper uptake by vesicles is temperature dependent, and solubilization with Triton X-100 results in a loss of accumulative capacity. Increasing osmotic pressure resulted in a decrease in copper levels in the vesicles at equilibrium, showing that uptake--as opposed to binding by the vesicles--occurred. Uptake by vesicles is concentration dependent, with evidence for cooperation in the uptake sites. The substrate concentration yielding 10% maximum uptake was 4.01 +/- 0.5 mumol/L, maximum uptake was 10.8 +/- 0.4 nmol/Cu/mg protein.min and the n value was 1.5 +/- 0.2. In contrast, uptake by cells showed no cooperation (n = 1.09 +/- 0.06) and a significantly higher apparent Michaelis-Menten constant (17.4 +/- 1.3 mumol/L). As expected, the maximum uptake was lower in the hepatocytes (1.82 +/- 0.08 nmol/mg protein.min). Albumin, N-ethylmaleimide and zinc all inhibited uptake in vesicles and in hepatocytes, and the degrees of inhibition were similar in both types of preparation. Vitamin C stimulated uptake in both vesicles and hepatocytes; again, there was a correlation between the increase in uptake at different concentrations. However, cadmium inhibited uptake and nickel stimulated uptake in vesicles and neither metal had any effect in hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Trace element-gene interactions

For many of the genes encoding proteins involved in the transport, storage, and function of the trace elements, expression is regulated by the availability of the elements concerned. This control is exercised through a variety of mechanisms, including metal-activated transcription factors, modified usage of stop codons, and use of secondary structure within mRNA to regulate its translation and stability. Two widely represented groups of transcription factors, often classed as zinc-finger proteins, depend on constituent zinc ions for their activity. In addition, the sensitivity of growth and fetal development to the lack of zinc is hypothesized to relate to a requirement for the element during certain critical alterations in gene expression. The evidence for this and possible underlying mechanisms is examined.

Mechanisms of caeruloplasmin biosynthesis in normal and copper-deficient rats

To examine the mechanisms of holo-caeruloplasmin biosynthesis, we measured the serum caeruloplasmin concentration and oxidase activity, hepatic caeruloplasmin mRNA content and hepatocyte caeruloplasmin biosynthesis and secretion in normal and copper-deficient rats. Copper deficiency resulted in a near-complete loss of serum caeruloplasmin oxidase activity, yet only a 60% reduction in serum caeruloplasmin concentration and no change in the abundance of hepatic caeruloplasmin mRNA or the rate of caeruloplasmin biosynthesis. Both interleukin-1 alpha and lipopolysaccharide increased hepatic caeruloplasmin mRNA content and caeruloplasmin biosynthesis in normal and copper-deficient animals, but neither mediator increased caeruloplasmin oxidase activity in the copper-deficient group. Pulse-chase studies in primary hepatocytes from normal and copper-deficient rats revealed that the secretory rates for newly synthesized caeruloplasmin were identical, despite little or no holo-caeruloplasmin synthesis in hepatocytes of copper-deficient rats. We conclude that hepatocyte copper content has no effect on hepatic caeruloplasmin-gene expression or caeruloplasmin biosynthesis and that the incorporation of copper into newly synthesized caeruloplasmin is not a rate-limiting step in the biosynthesis or secretion of the apoprotein from rat hepatocytes.